The present invention relates to a keyswitch assembly, and more particularly, to a keyswitch assembly suitable for use on a thin keyboard for a portable word processor, a portable personal computer or the like.
A conventional keyswitch assembly for use on such a keyboard has a key integrally provided with a key stem, a holder plate, and a switching member. The holder plate is provided with a key support or guide having a hole which receives the stem of the key to guide the key for vertical movement. The switching member is provided below the key stem. If the key is depressed, the lower end of the key stem presses the switching member for performing switching.
A keyswitch assembly having a large key, such as a space key and a return key, is provided with a mechanism for maintaining the key in a level position when the key is depressed regardless of the actual finger depressing position on these large keys. Such keyswitch assemblies are disclosed in U.S. Pat. No. 4,580,022 and U.S. Pat. No. 4,902,862.
In the keyswitch assembly disclosed in the U.S. Pat. No. 4,580,022, two scissors-like members serving as support levers are disposed below a key member for supporting the same. Two levers of each of the scissors-like members are pivotally linked to a shaft. A plurality of pins are disposed at extremities of the levers. When the key member is depressed, these pins are slidingly moved in a horizontal direction along the rear surface of the key member and along the upper surface of a base plate of the keyboard. With the structure, when the key member is depressed, the key member is moved downwardly while maintaining its level position. In accordance with this downward movement, a key depressing member suspended from the key member is slidingly guided by the key support, and buckles a rubber spring and presses the switching member disposed therebelow for switching action.
A key switch assembly disclosed in the U.S. Pat. No. 4,902,862 is the same in basic construction as the keyswitch assembly disclosed in the '022 patent, and is characterized in that the key member can be easily connected to and removed from the scissors-like support levers.
In the conventional keyswitch assemblies disclosed in the above U.S. patents, the key member is maintained in a level position regardless of the position at which pressure is applied to the key member, when the key member is moved vertically, even if the key member is a large key member such as the space key. In any of these prior art keyswitch assemblies, the stem for compressing the switching member or the key depressing member is guided for vertical sliding movement by the guide member in compressing the switching member.
Still another conventional keyswitch assembly is provided with a coil spring interposed between the lower surface of the key and the upper surface of the rubber spring in order to enhance touch of keys. By the replacement of the coil springs, the touch can be altered.
Recent progressive reduction in size and thickness of word processors and personal computers requires reduction in size and thickness of keyboards to be incorporated thereinto. On the other hand, the stroke of the keys of keyboards must be sufficiently large to facilitate keystroke operation and to secure a reliable keystroke. However, sufficiently large stoke of the keys is not obtainable in the prior art keyswitch assemblies.
When reducing the thickness of keyboard provided with the prior art keyswitch assemblies, the length of a sliding portion of the key stem in sliding engagement with the guide member must be reduced. However, if the length of the sliding portion of the key stem is reduced, the key is liable to tilt relative to the guide member, and consequently, the key stem is liable to slide awkwardly in the guide member, for example, local excessive sliding contact occurs when the key is depressed. On the other hand,if the length of the sliding portion of the stem in engagement with the guide member is increased to ensure smooth movement of the key, the stroke of the key is reduced.
If the key stem slides awkwardly in the guide member when the key is depressed, noise is generated and the keystroke operability may be degraded. The misalignment of the key stem with the guide member does not occur frequently if the key is always depressed in the central portion thereof. Hence, it may be possible to obviate the misalignment by reducing upper surface area of the key so that the key is always depressed in the central portion thereof. However, small area of the upper surface of the key may deteriorate facility in keystroke operation.
The keyswitch assemblies disclosed in the foregoing U.S. Patents are not intended to enable the reduction of the thickness of the keyboard. Since the key depression member for compressing the switching member protrudes downwardly from the key member, it would be difficult to form those keyswitch assemblies in a relatively small thickness. Further, the key provided with the key depression member has a complicated shape and hence increases the cost of the keyswitch assemblies.
Furthermore, since the pins formed at the extremities of the scissors-like support levers slide horizontally along the rear surface of the key member and the upper surface of the base plate, respectively, the position of the key with respect to horizontal direction is indefinite. Consequently, the key is likely to be dislocated horizontally when depressed, and the key stem is unable to operate the switching member reliably.
If the coil spring is additionally disposed on the upper surface of the rubber spring in an attempt to improve the touch of the keys, the key switch assembly becomes inevitably thick. In order to alter the touch of the keys, each of the coil springs must be replaced by new spring after the key is removed, which works require labors and skill.